1. Field of the Invention
This invention relates to the field of fuel cells. More specifically, the invention comprises a novel fuel cell construction using two electrolytes per unit and internal electrical connections rather than edge connections.
2. Description of the Related Art
Although the operation of a conventional fuel cell is well understood by those skilled in the art, some explanation of the terminology of the components and the operation of the assembly will aid the reader's understanding. FIG. 1 shows a prior art fuel cell assembly having only one cell. In general, a fuel cell includes two reactants that are physically separated by some type of electrolyte. There are many types of fuel cells, and they are often categorized according to the type of electrolyte used. The particular example shown in FIG. 1 uses a proton exchange membrane (“PEM”) for electrolyte 14. It is commonly called a “PEM” fuel cell.
The proton exchange membrane (“PEM”) is flanked by a pair of porous electrodes. Anode 12 is located on a first side of the PEM and cathode 16 is located on the other side. A gas diffusion layer is also located on each side of the PEM. Hydrogen diffusion layer 30 is located on the left side in the orientation of FIG. 1. Oxygen diffusion layer 32 is located on the right side. Hydrogen inlet 18 feeds gaseous hydrogen into the hydrogen diffusion layer, while oxygen inlet 20 feeds gaseous oxygen into the oxygen diffusion layer. A “diffusion layer” may be created using many known techniques. A diffusion layer is commonly created using a sealed manifold containing the particular flowing reactant gas.
Negative charge collector 54 is in contact with hydrogen diffusion layer 30 while positive charge collector 56 is in contact with oxygen diffusion layer 32. External conductor path 22 electrically connects the negative charge collector to the positive charge collector. Electrical load 24 is placed in this conductor path. A typical goal for the operation of such a fuel cell is the creation of an electrical current in the external conductor path which is used to provide energy to electrical load 24.
The operation of the exemplary PEM fuel cell of FIG. 1 will now be described in detail. Two electrochemical reactions are required for the operation of the fuel cell—an anode reaction and a cathode reaction. The anode reaction for a PEM cell may be written as:H2→2H++2e−
The cathode reaction may be written as:
                    1        2            ⁢              O        2              +          2      ⁢              H        +              +          2      ⁢                          ⁢              e        -              →            H      2        ⁢    O  
The overall reaction may then be written as:
            H      2        +                  1        2            ⁢              O        2              →            H      2        ⁢    O  
Catalysts are generally required to facilitate the reactions. The anode catalyst is typically nickel or platinum powder deposited in a very thin layer on the porous anode. Flow channeling devices are typically used to force the gaseous hydrogen to flow along a long, serpentine path so that it remains in contact with the catalyst for an extended period. The catalyst facilitates the splitting of the diatomic hydrogen into free hydrogen nuclei (free protons) and free electrons.
The proton exchange membrane is configured to allow the passage of free protons (the hydrogen nuclei) but to prevent the passage of the free electrons. Thus, the hydrogen nuclei pass through the PEM but the free electrons cannot. Instead, the free electrons are collected by negative charge collector 54 and forced to flow through external conductor path 22. The free electrons pass through positive charge collector 56 and ultimately to cathode 16.
At the cathode the free electrons combine with the oxygen and the hydrogen nuclei passing through the PEM to form water. A catalyst is generally used for the cathode reaction as well, with platinum being a common example.
Many other components are included in actual PEM fuel cell designs. These include:
(1) Channels for removing the water formed at the cathode;
(2) Devices for maintaining the appropriate conditions for the PEM;
(3) Cooling devices for removing excess heat produced by the electrochemical reactions; and
(4) Gas throttling valves for controlling the output of the fuel cell.
FIG. 2 depicts a different type of fuel cell in which a solid oxide is used for electrolyte 14. This type is often referred to as a “SO” fuel cell. Most commonly a yttria-stabilized zirconia is used as the electrolyte. SO fuel cells operate at relatively high temperatures (800 to 1,000 degrees centigrade). The anode and cathode reactions differ from the reactions existing in a PEM cell. The anode reaction for an SO cell may be written as:H2+O2−→H2O+2e−
The cathode reaction may be written as:
                    1        2            ⁢              O        2              +          2      ⁢                          ⁢              e        -              →      O          2      -      
The overall reaction may again be written as:
            H      2        +                  1        2            ⁢              O        2              →            H      2        ⁢    O  
The current vector in the device is the same as for the PEM cell, but of course the ions move in the opposite direction. In the SO fuel cell, an ionized oxygen atom moves from the cathode side of the electrode toward the anode side. Thermal management and water removal may also pose differing challenges. However, the conceptual operation of PEM cell and the SO cell are grossly similar.
As those skilled in the art will know, the voltage produced by an individual cell such as shown in FIG. 1 or 2 is quite small—typically in the range of 0.7V. The electrical current produced by each cell is a function of charge accumulation. Thus, one may linearly increase the current produced by increasing the surface area of the components. Larger anodes and cathodes produce more current. The voltage, however, is fixed by the electrochemical reactions themselves.
The low voltage produced by a single cell is not very useful, particularly if it must be conveyed for any significant distance. The solution to this problem is to “stack” multiple cells together in the same way that single battery cells are stacked to increase voltage. FIG. 3 provides a conceptual depiction of a “stacked” arrangement of two SO fuel cells. Cell “A”—shown on the left—is identical to the single cell shown in FIG. 2. Cell “B”—shown on the right—is also identical.
However, the external electrical circuits have been reconfigured to stack the voltage produced. The reader will observe that external conductor path 22 has been connected from negative charge collector 54 on Cell A to positive charge collector 56 on Cell B. Linking circuit 34 has been used to connect negative charge collector 54 on Cell B to positive charge collector 56 on Cell A. As a result, free electrons created by the anode reaction in Cell A are transported by external conductor path 22 to the cathode reaction in Cell B (where they react with diatomic oxygen to form oxygen atoms). Free electrons formed by the anode reaction in Cell B are transported via linking circuit 34 to the cathode reaction in Cell A.
Those skilled in the art will quickly realize that linking circuit 34 may be eliminated by simply pushing positive charge collector 56 of Cell A and negative charge collector 54 of Cell B together (providing that the mating surfaces of the charge collectors are suitably conductive). This is in fact what is done in most fuel cell “stack” assemblies. FIG. 4 shows this configuration, with the exception of separator plate 36 being substituted for a pair of mating charge collector plates. Separator plate 36 is made of conductive material. It must be able to survive exposure to the charged gaseous oxygen environment on one side and the charged gaseous hydrogen environment on the other.
Looking at the configuration of FIG. 4, the reader will realize that the two cell stack shown could be expanded to three cells, four cells, or any desired additional number. FIG. 5 shows a prior art design using this approach in which six cells (A through F) have been stacked in series. If each cell produces 0.7 volts, then a stack of six such cells will (neglecting losses) produce 4.2 volts. Thus, a fuel cell designer using the prior art approach is able to: (1) produce increasing current by increasing the surface area of the components, and (2) produce increasing voltage by increasing the number of individual fuel cells in the stack.
Additionally, the mechanical arrangement allows the entire assembly to be held together using sets of tie rods that pass through the assembly. The ends of the tie rods are threaded and nuts are tightened on these threaded ends to clamp the stack together. Suitable insulating and sealing components are of course added to the tie rods so that the reactants don't leak and no electrical short circuits are created.
The back-to-back stack approach does, however, have some recognized shortcomings. These include, among others: (1) The separator plate must be made of a material that can resist the oxidizing and reducing environments, (2) The tightly packed nature causes heat dissipation problems; (3) The tightly packed nature causes problems with feeding in the reactant gases.